In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using a number of field devices distributed at key locations in the industrial process. Field devices may operate in a variety of different installations. Examples of process installations include petroleum, pharmaceutical, chemical, pulp and other processing installations. These field devices are communicatively coupled to control circuitry in a control room.
Field devices are used by the process control measurement industry for a variety of purposes. Usually, such devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and be able to withstand climatological extremes of temperature, humidity, vibration and mechanical shock. Field devices also typically operate on relatively low power. For example, some field devices are currently available that receive all of their operating power from a known 4-20 mA loop.
The environment within which the field devices operate can sometimes be highly volatile. Some environments may be so volatile that an errant spark or a sufficiently high surface temperature of an electrical component could cause the environment to ignite and generate an explosion. In order to ensure that such situations do not occur, intrinsic safety specifications have been developed. Compliance with an intrinsic safety requirement helps ensure that even under fault conditions, the circuitry or device itself cannot ignite a volatile environment. One specification for an intrinsic safety requirement is set forth in: APPROVAL STANDARD INTRINSICALLY SAFE APPARATUS AND ASSOCIATED APPARATUS FOR USE IN CLASS I, II AND III, DIVISION 1 HAZARDOUS (CLASSIFIED) LOCATIONS, CLASS NUMBER 3610, promulgated by Factory Mutual Research October 1988. Adaptations to comply with additional industrial standards such as Canadian Standards Association (CSA) and the European CENELEC standards are also contemplated.
Traditionally, field devices have been coupled to the process communication system (such as a control room) by virtue of physical conductors. Such wired connections have not only provided power to the field devices, but have also provided a route for communication. One limitation with wired field devices is that installation can sometimes be labor intensive since wires must be run to each physical location of a field device.
More recently, field devices have emerged that employ wireless communication to communicate with the control room and/or other suitable devices. These wireless field devices are generally provided with an internal power source, such as a battery, that can provide operating power for the wireless field device for a period of years.
The use of wireless technology in the industrial process industry has created a need for a way to locally power wireless transmitters. Batteries are one way that local power can be provided. However, one problem with batteries lies in the decision about how and where to locate the batteries within the wireless field device. It is desirable for battery replacement to be easy for field technicians. It is not only desirable that the battery itself be physically easy to replace, but also desirable in cases where the field device is located in a hazardous area. Specifically, it is desirable to be able for the technician to replace the battery without the need for obtaining a hot work permit. This means that the battery can be replaced in an area where there is flammable gas present without risk of igniting that gas.
Another difficulty regarding the use of batteries in wireless field devices is due to the fact that the capacity of the batteries is generally related to the physical size of the battery itself. Thus, the size of batteries located inside a device is often constrained by the housing of the device itself. There is generally little flexibility in offering large-capacity battery packs to support longer transmitter life and/or higher update rates or heavy load due to network configuration.
Accordingly, there is a need for an intrinsically safe battery pack that can power field devices and be replaceable in the field without requiring hot work permits. Further, it is desirable that field device power be provided in such a way that larger-capacity batteries and/or higher power demands can be accommodated.